Lithium secondary battery

ABSTRACT

A lithium secondary battery with an electrolyte containing one or more alkai metal salts, one or more non-aqueous solvents and immobilized by a polymer selected from cellulose acetates, cellulose acetate butyrates, cellulose acetate propionates, polyvinylidene fluoride-hexafluoropropylenes and polyvinylpyrrolidone-vinyl acetates, the polymer preferably being used in an amount of at most 15% by weight based on the weight of the salts, solvents and polymer of the electrolyte system, with the proviso that in the case of polyvinlidene fluoride-hexafluoropropylenes, the polymer is present in an amount of at most 12% by weight based on the weight of the salts, solvents and polymer of the electrolyte system. The immobilized electrolyte does not cause problems with respect to leakage from the cell compartment and the elctrolyte also high conductivity implying a capacity utilization more closely approaching the utilization observed for batteries using liquid electrolyte. The electrolyte is also electrochemically stable.

This invention relates to a lithium secondary battery, and particularlyto the electrolyte used therein and to the preparation of the battery.

Recent developments in electrochemical technology have provided systemssuch as primary and secondary lithium batteries of high specific energyper unit of volume, typically in the range 175-250 Wh/l.

Such secondary batteries are typically based on negative electrodestructures of metallic lithium, alloys thereof or on carbons of highlithium intercalation capacity. The positive electrode structures aretypically based on transition metal oxides. The electrolyte comprisesone or more non-aqueous solvents, and one or more lithium-salts.

The fact that the electrolyte is a liquid at the battery operationtemperature may cause safety problems such as electrolyte leakage fromthe cell compartment. Upon reaction with oxygen and water in theatmosphere, severe corrosion of the battery may occur.

Several attempts have been made to solve this safety issue of lithiumbased batteries. The traditional approach has been solid polymerelectrolytes, i.e. electrolyte structures which are based on ionicconduction within a solid polymer network. Such polymer electrolytesprovide batteries of high safety, as no electrolyte leakage can takeplace.

Polymer electrolytes are described in a number of patents and patentapplications, including the following:

EP 724,305 A1 to Sony Corporation, which describes gel electrolytes of apolymer having a side chain to which at least one nitrile group isbonded.

U.S. Pat. No. 5,240,790 to Alliant Techsystems Inc., which covers agelled electrolyte comprising polyacrylonitrile, preferably of arelative concentration of 12-22 mole percent.

U.S. Pat. No. 5,589,295 to Derzon et al, which describes a thin filmelectrolyte with a polymeric gel-former selected from the group ofpolyacrylonitrile and polyvinylidenefluoride.

The drawback of batteries based on such solid polymer electrolytes isreduced capacity and power capability, especially at low temperature.Compared to liquid electrolytes, the conductivity of solid polymerelectrolytes is lower, mainly due to reduced ionic mobility. Further,the activation energy for the ionic migration process is higher than forthe liquid electrolytes, implying strong conductivity variation withtemperature and significantly reduced low-temperature performance. Thecapacity and power capability are strongly dependent on the electrolyteconductivity; at low conductivity high internal impedance implies highlosses and reduced capacity accessability.

Therefore a need exists for secondary lithium batteries based on polymerelectrolyte systems, which combine the demands for high safety and highconductivity.

An object of the present invention is to provide a lithium secondarybattery which avoids problems with respect to electrolyte leakage fromthe cell compartment but which also provides high conductivitysufficient for full capacity utilisation, i.e. which does not imply thesame reduction in capacity utilisation compared to lithium secondarybatteries based on liquid electrolytes that is associated with knownpolymer electrolytes.

The present invention provides a lithium secondary battery comprising animmobilized electrolyte containing one or more alkali metal salts, oneor more non-aqueous solvents and an immobilizing polymer, wherein theimmobilizing polymer is selected from the group consisting of celluloseacetates, cellulose acetate butyrates, cellulose acetate propionates,polyvinylidene fluoride-hexafluoropropylenes andpolyvinypyrrolidone-vinyl acetates, with the proviso that in the case ofpolyvinylidene fluoride-hexafluoropropylenes, the polymer is present inan amount of at most 12% by weight based on the weight of the salts,solvents and polymer of the electrolyte system.

Surprisingly, it has been found that lithium secondary batteries whichcomprise as an electrolyte component an immobilising polymer selectedfrom the group of cellulose acetates, cellulose acetate butyrates,cellulose acetate propionates, polyvinylidenefluoride-hexafluoropropylenes and polyvinylpyrrolidone-vinyl acetatesdoes not cause problems with respect to electrolyte leakage from thecell compartment. Further, the electrolytes of such batteries have ahigh conductivity implying a capacity utilisation more closelyapproaching the utilisation observed for batteries using liquidelectrolyte. Still further, the electrolytes of such batteries areelectrochemically stable, i.e. they are not oxidised or reduced evenunder the redox conditions observed in high voltage lithium batteries.

Compared to the known technology on polymer electrolyte based secondarybatteries referred to above, the polymers used according to theinvention are generally present in relatively small amounts, preferablyat most 15% by weight based on the weight of the salts, solvents andpolymer of the electrolyte system,

According to one embodiment of the invention, the cellulose polymersused according to the present invention will usually be present in anamount ranging from 0.1% to 10% by weight of the complete electrolytesystem, i.e. the total weight of salts, solvents and polymer, preferably1% to 8% by weight, more preferably 2% to 5% by weight.

In another embodiment of the invention, the polyvinylidenefluoride-hexafluoropropylenes are present in an amount of from 1% to 12%by weight of the complete electrolyte system, i.e. the weight of salts,solvents and polymer, preferably 2 to 10% by weight, more preferably 4%to 8% by weight. In a still further embodiment of the invention thepolyvinylpyrrolidone-vinyl acetates will usually be present in an amountfrom 1% to 15% by weight of the complete electrolyte system, i.e. thetotal weight of salts, solvents and polymer, preferably 3% to 12% byweight, more preferably 5% to 10% by weight.

WO 97/12409 to Valence Technology describes “viscosifiers” forelectrolytes, which are selected from the group of polyethylene oxide,polypropylene oxide, carboxymethylcellulose and polyvinylpyrrolidone.Although this patent specification describes the use of “viscosifiers”based on a cellulose compound and a polyvinylpyrrolidone, it does notdescribe the specific immobilising agents cellulose acetates andpolyvinylpyrrolidone-vinyl acetates used according to the presentinvention.

U.S. Pat. No. 5,296,318 to Bell Communications Research discloses anelectrolyte comprising a self-supporting film of a copolymer ofvinylidene fluoride and hexafluoropropylene. Such copolymer ispreferably present in the electrolyte in an amount corresponding to 30to 80% of the electrolyte. Although the patent describes the use ofpolyvinylidene fluoride-hexafluoropropylene, it does not describe orsuggest the use of this material in amounts as small as 12% or less byweight of the electrolyte system.

The immobilizing properties of the polymers used according to thepresent invention may be improved by crosslinking.

In a preferred embodiment, the immobilizing properties of the cellulosepolymers used according to the invention are improved by crosslinking.In this embodiment, cellulose acetates, cellulose acetate butyrates andcellulose acetate propionates, preferably of high hydroxyl content, forexample 3% by weight or more, are mixed with monomers or oligomers,which bear functional groups, and which can be crosslinked upon heatcuring or upon exposure to UV-light or electron beams. Such monomers andpolymers are preferably selected from urea formaldehyde, melamine andpolyisocyanate polymers.

In another preferred embodiment of the invention, the electrolyte of thelithium secondary battery comprises, in addition to the immobilisingpolymer, one or more solvents selected from organic carbonates,lactones, esters and glymes, more preferably selected from the groupsof:

(a) alicyclic carbonates represented by the following general formula:

—C(═O)—O—CR₁R₂—[CR₃R₄]_(m)—CR₅R₆—O—,

wherein each of R₁, R₂, R₃, R₄, R₅ and R₆ independently representshydrogen or a C₁-C₄ alkyl group and m is 0 or 1, preferably ethylenecarbonate or propylene carbonate;

(b) aliphatic carbonates represented by the general formulaR₇[OC(O)]_(p)OR₈, wherein each of R₇ and R₈ independently represents aC₁-C₄ alkyl group, and p is an integer equal to 1 or 2, preferablydimethyl carbonate or diethyl carbonate;

(c) lactones in the form of cyclic esters represented by the generalformula:

—C(═O)—CR₉R₁₀—CR₁₁R₁₂—[CR₁₅R₁₆]_(r)—CR₁₃R₁₄—O—

wherein each of R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, R₁₅ and R₁₆ independentlyrepresents hydrogen or a C₁₋₂ alkyl group and r is 0 or 1, preferablyγ-valerolactone or γ-butyrolactone;

(d) esters represented by the formula R₁₇[C(O)]OR₁₈[OR₁₉]_(t), whereineach of R₁₇, R₁₈ and R₁₉ independently represent hydrogen or a C₁-C₂alkyl group, and t is 0 or an integer equal to 1 or 2, preferably anacetate, more preferably (2-methoxyethyl)-acetate or ethyl acetate;

(e) glymes represented by the general formula R₂₀O(R₂₁O)_(n)R₂₂, inwhich each of R₂₀ and R₂₂ independently represents a C₁₋₂ alkyl group,R₂₁ is —(CR₂₃R₂₄CR₂₅R₂₆)— wherein each of R₂₃, R₂₄, R₂₅ and R₂₆independently represents hydrogen or a C₁-C₄ alkyl group, and n is aninteger from 2 to 6, preferably 3, R₂₀ and R₂₂ preferably being methylgroups, R₂₃, R₂₄, R₂₅ and R₂₆ preferably being hydrogen or C₁-C₂ alkylgroups, more preferably hydrogen.

Such solvents may contribute further to the electrochemical stabilityand ionic conductivity of the electrolyte of the battery.

Any salt commonly employed as an ion-conductive salt in batteries may beused in the electrolyte system according to the invention. Preferably,however, the salt is an alkali metal salt of ClO₄ ⁻, CF₃SO₃ ⁻, AsF₆ ⁻,PF₆ ⁻ or BF₄ ⁻, or any mixture of such alkali metal or ammonium salts,more preferably LiAsF₆, LiCF₃SO₃, LiPF₆ or LiBF₄ or any mixture thereof.Those salts are preferably present in the electrolyte solvents in aconcentration from 0.01M to 2.5M, more preferably 0.1M to 1.5M.

In a preferred embodiment of the battery of the invention theelectrolyte is incorporated into a porous separator.

Thus the immobilized electrolyte according to the invention isoptionally incorporated into a separator which is a porous structuremade from a polymer, preferably polyethylene, polypropylene,polycarbonate, cellulose or cellulose derivate, or made from a glassfibre material e.g. boron silicate glass fibre material.

The separator acts as a matrix which confines the physical dimensions ofthe electrolyte system, thereby enabling the production of thin,self-sustaining and uniform electrolyte membranes. The separator ispreferably a woven or non-woven structure having a pore size in therange of 10×10 nm to 1×1 mm and a thickness of 10-100 μm, preferably10-25 μm. More specifically, the size of the pores can be as in amicroporous film (e.g. a Celgard separator) or up to 1×1 mm as in awoven net having a mesh of this size.

The present invention also provides a simple and economicallyadvantageous method for the preparation of the lithium secondary batteryof the invention. In general terms, this method comprise the steps ofpreparing the immobilized electrolyte by mixing the solvents (where morethan one solvent is used), dissolving the salt(s) in the solventmixture, adding an immobilizing agent to the solution, and optionallycrosslinking the immobilizing agent.

Thus according to another aspect the present invention provides a methodfor the preparation of a lithium secondary battery as defined above,comprising the steps of:

mixing the solvents in case the electrolyte comprises more than onesolvent,

dissolving the salt(s) in the solvent(s) to provide an organicelectrolyte,

adding the immobilizing polymer and optionally monomers or oligomershaving one or more polymerisable functional groups, to the organicelectrolyte,

if monomers or oligomers are added, inducing polymerisation of thesemonomers or oligomers,

sandwiching the immobilized organic electrolyte between a positiveelectrode laminate and a negative electrode laminate to form thebattery.

Optionally the battery is wound or folded as it is known in the art.

According to a still further aspect, the present invention also providesthe use of a polymer selected from the group consisting of celluloseacetates, cellulose acetate butyrates, cellulose acetate propionates,polyvinylidene fluoride-hexafluoropropylenes andpolyvinypyrrolidone-vinyl acetates as immobilizing agent for anelectrolyte in a lithium secondary battery, with the proviso that in thecase of polyvinylidene fluoride-hexafluoropropylenes, the polymer ispresent in an amount of at most 12% by weight based on the weight of thesalts, solvents and polymer of the electrolyte system.

The present invention is illustrated by the following non-limitingexamples together with a comparative example.

EXAMPLE 1

A lithium secondary battery was prepared from a negative electrodelaminate of a polymer bound carbon coated onto a copper currentcollector, a positive electrode laminate of a polymer bound lithiummanganese oxide spinel coated onto an aluminium current collector, andan electrolyte sandwiched between the electrode laminates.

The carbon was R-LIBA-A (product of Timcal, Switzerland). The lithiummagnesium oxide spinel was prepared by a solid state reaction at 800° C.from Li₂CO₃ and MnO₂ and had a specific capacity of 120 mAh/g. In thecase of both electrodes, the polymeric binder was EPDM (ethylenepropylene diene polymethylene).

The electrolyte was prepared by:

mixing equimolar amounts of propylene carbonate (PC) and ethylenecarbonate (EC)

adding LiBF₄ to obtain a 1M solution of LIBF₄ in PC/EC

adding cellulose acetate butyrate (CAB) to the solution to obtain a 3%by weight solution of CAB in 1M LiBF₄ in PC/EC.

incorporating the above CAB electrolyte in a microporous polyethyleneseparator

The battery prepared had an active electrode area of 365 cm² and,subsequent to charging to 4.2V, an internal impedance of 49 mΩ at 1 kHz.When cycled between 4.2V and 2.5V at 500 mA, the battery displayed aninitial capacity of 358 mAh. After 400 cycles, the capacity was 299 mAh,say 84% of the initial capacity. At 1.25 A discharge rate, an initialcapacity of 210 mAh was observed.

Upon nail penetration (Ø=5 mm, F=6000N) the battery short-circuited,however, no leakage of electrolyte was observed on the surface of thebattery upon visual inspection.

Comparative Example

A lithium secondary battery was prepared following the same procedure asdescribed in the above example 1, however, 1M LIBF₄ in PC/EC was used,i.e. no cellulose acetate butyrate was added to the electrolytesolution.

Such a battery, based on the same electrodes as in example 1 and havingthe same dimensioanl characteristics as the battery of example 1, had aninternal impedance of 49 mΩ at 1 kHz. When cycled between 4.2V and 2.5Vat 500 mA, the battery displayed an initial capacity of 408 mAh. After400 cycles, the capacity was 343 mAh, say 84% of the initial capacity.At 1.25 A discharge rate, an initial capacity of 360 mAh was observed.

Upon nail penetration (Ø=5 mm, F=6000N) the battery short-circuited.Leakage of electrolyte was observed on the surface of the battery uponvisual inspection.

EXAMPLE 2

A lithium secondary battery was prepared following the same procedure asdescribed in the above example 1, however, 6% of polyvinylidenefluoride-hexafluoropropylene was substituted for the 3% CAB of example1.

Upon nail penetration (Ø=5 mm, F=6000N) the battery short-circuited,however, no leakage of electrolyte was observed on the surface of thebattery upon visual inspection.

EXAMPLE 3

A lithium secondary battery was prepared following the same procedure asdescribed in the above example 1, however, 8% ofpolyvinylpyrrolidone-vinyl acetate was substituted for the 3% CAB ofexample 1.

Upon nail penetration (Ø=5 mm, F=6000N) the battery short-circuited,however, no leakage of electrolyte was observed on the surface of thebattery upon visual inspection.

What is claimed is:
 1. A lithium secondary battery comprising animmobilized electrolyte containing one or more alkali metal salts, oneor more non-aqueous solvents and an immobilizing polymer, wherein theimmobilizing polymer is selected from the group consisting of celluloseacetates, cellulose acetate butyrates, cellulose acetate propionates,and polyvinylidene fluoride-hexafluoropropylenes with the proviso thatin the case of polyvinylidene fluoride-hexafluoropropylenes, the polymeris present in an amount or at most 12% by weight based on the weight ofthe salts, solvents and polymer of the electrolyte system.
 2. A lithiumsecondary battery according to claim 1, wherein the immobilizing polymeris present in an amount of at most 15% by weight based on the weight ofthe salts, solvents and polymer of the electrolyte system.
 3. A lithiumsecondary battery according to claim 1, wherein the polymer is selectedfrom the group consisting of cellulose acetates, cellulose acetatebutyrates and cellulose acetate propionates.
 4. A lithium secondarybattery according to claim 3, wherein the polymer is present in anamount of from 0.1% to 10% by weight based an the weight of the salts,solvents and polymer of the electrolyte system.
 5. A lithium secondarybattery according to claim 3, wherein the cellulose polymer has ahydroxyl content of 3% by weight or more.
 6. A lithium secondary batteryaccording to claim 1, wherein the polymer is mixed with monomers oroligomers selected from urea formaldehyde, melamine and polyisocyanatepolymers.
 7. A lithium secondary battery according to claim 6, whereinthe polymer is crosslinked upon exposure to heat, light or electronradiation.
 8. A lithium secondary battery according to claim 1, whereinthe polymer is polyvinylidene fluoride-hexafluoropropylene.
 9. A lithiumsecondary battery according to claim 8, wherein the polymer is presentin an amount of from 1% to 12% based on the weight of the salts,solvents and polymer of the electrolyte system.
 10. A lithium secondarybattery according to claim 1, wherein the polymer is present in anamount of from 1% to 15% by weight based on the weight of the salts,solvents and polymer of the electrolyte system.
 11. A lithium secondarybattery according to claim 1, wherein the electrolyte comprises one ormore of the following solvents (a) to (e): (a) alicyclic carbonatesrepresented by the following general formula:—C(═O)—O—CR₁R₂—[CR₃R₄]_(m)—CR₅R₆—O—, wherein each of R₁, R₂, R₃, R₄, R₅and R₆ independently represents hydrogen or a C₁-C₄ alkyl group and m isan 0 or 1; (b) aliphatic carbonates represented by the general formulaR₇[OC(O)]_(p)OR₈, wherein each of R₇ and R₈ independently represents aC₁-C₄ alkyl group, and p is an integer equal to 1 or 2; (c) lactones inthe form of cyclic esters represented by the general formula: —C(═O)—CR₉R₁₀—CR₁₁R₁₂—[CR₁₅R₁₆]_(r)—CR₁₃R₁₄—O— wherein each of R₉, R₁₀,R₁₁, R₁₂, R₁₃, R₁₄, R₁₅ and R₁₆ independently represents hydrogen or aC₁₋₂ alkyl group and r is 0 or 1; (d) esters represented by the formulaR₁₇[C(O)]OR₁₈[OR₁₉]_(t), wherein each of R₁₇, R₁₈ and R₁₉ independentlyrepresents hydrogen or a C₁-C₂ alkyl group, and t is 0 or an integerequal to 1 or 2; (e) glymes represented by the general formulaR₂₀O(R₂₁O)_(n)R₂₂, in which each of R₂₀ and R₂₂ independently representsa C₁₋₂ alkyl groups, R₂₁ is —(CR₂₃R₂₄CR₂₅R₂₆)— wherein each of R₂₃, R₂₄,R₂₅ and R₂₆ independently represents hydrogen or a C₁-C₄ alkyl groups,and n is an integer from 2 to
 6. 12. A lithium secondary batteryaccording to claim 1, wherein the electrolyte comprises one or moresalts selected from the group of alkali metal or ammonium salts of ClO₄⁻, CF₃SO₃ ⁻, AsF₆ ⁻, PF₆ ⁻ or BF₄ ⁻.
 13. A battery according to claim12, wherein the salts are present in the electrolyte solvent(s) in aconcentration from 0.01M to 2.5M.
 14. A lithium secondary batteryaccording to claim 1, wherein the electrolyte is confined in a separatorconsisting of a porous structure made of a polymer or made of a glassfibre material.
 15. A lithium secondary battery according to claim 14,wherein the separator is a woven or non-woven structure having a poresize in the range of 10×10 nm to 1×1 mm.
 16. A lithium secondary batteryaccording to claim 14, wherein the separator has a thickness of 10-100μm.
 17. A method for the preparation of a lithium secondary batteryaccording to claim 1, comprising the steps of: mixing the solvents incase the electrolyte comprises more than one solvent, dissolving thesalt(s) in the solvent(s) to provide an organic electrolyte, adding theimmobilizing polymer and optionally monomers or oligomers having one ormore polymerisable functional groups, to the organic electrolyte, ifmonomers or oligomers are added, inducing polymerisation of thesemonomers or oligomers, sandwiching the immobilized organic electrolytebetween a positive electrode laminate and a negative electrode laminateto form the battery.
 18. A method of immobilizing an electrolyte in alithium secondary battery comprising an electrolyte system whichincludes salts, solvents and polymer, said method comprising combiningthe electrolyte with a polymer selected from the group consisting ofcellulose acetates, cellulose acetate butyrates, cellulose acetatepropionates and polyvinylidene fluoride-hexafluoropropylenes with theproviso that when the polymer is polyvinylidenefluoride-hexafluoropropylenes, the polymer is present in an amount of atmost 12% by weight based on the weight of the salts, solvents andpolymer of the electrolyte system.